Carbon dioxide is accumulating in the atmosphere and it is suggested to cause the so-called greenhouse effect. Therefore, the reduction of carbon dioxide from atmosphere and emission sources is of great interest to mitigate global warming. At the same time, the source of carbon atoms in carbon dioxide largely stems from burning fossil fuels, causing the loss of natural carbon resources on Earth. One attractive solution for both problems is to use this great amount of carbon dioxide for its transformation into valuable compounds and chemical fuels such as methanol and its transformation products. This will allow to reduce the carbon dioxide concentration in the atmosphere as well as to close the carbon cycle which has an open end at present.
Methanol is one of the primary chemicals for industries, and a promising alternative for oil and natural gas, with applications in energy storage, fuel cells and the preparation of a variety of bulk chemicals such as formaldehyde, dimethyl ether, ethylene, propylene, gasoline and acetic acid. The synthesis of methanol from hydrogen and carbon dioxide or from mixtures of hydrogen, carbon dioxide, and carbon monoxide (the so-called syngas) using a solid catalyst is still attracting much interest due to its economical and environmental relevance, thus making carbon dioxide a valuable and renewable carbon source rather than a cause of global warming.
Catalytic methanol synthesis from hydrogen and carbon dioxide or syngas is a well-known exothermic reaction with limited equilibrium, usually requiring moderately high temperature and pressure. The process of methanol synthesis can be summarized as follows:

Methanol and carbon monoxide are produced competitively from carbon dioxide through methanol synthesis (equation (1)) and the reverse water gas shift reaction (equation (2) to the left). Therefore, it has been suggested that in order to increase the amount of methanol formed, a stoichiometric or superstoichiometric amount of hydrogen is required, i.e. a molar ratio of carbon dioxide to hydrogen equal to or higher than 1:3.
The first catalysts used in the commercial methanol synthesis were ZnO/Cr2O3, being the mixture enriched in ZnO. This process was operated at 350° C., and about 260 bar, by reacting hydrogen with a mixture of carbon dioxide and carbon monoxide (U.S. Pat. No. 1,569,775, and U.S. Pat. No. 1,558,559). This catalytic system suffers from deactivation and, in the process plant, a significant amount of the unreacted mixture of carbon oxides and hydrogen had to be recycled due to partial conversion of the oxides after one pass through the reactor.
Ipatieff and co-workers reported a process for methanol synthesis by reacting hydrogen with carbon dioxide, or with a mixture of carbon dioxide, and carbon monoxide, using a copper-alumina based catalyst with low GHSVs in the range of 900 to 1600 h−1. The copper-alumina catalyst was prepared by a deposition precipitation method comprising the precipitation of copper carbonate within a suspension of alumina followed by calcination of the dried precipitate at 240° C. Although this process has high conversions of carbon oxides in one pass through the catalyst bed and it is selective to methanol formation, the amount of methanol obtained following this process per unit of time is too low for commercial application. (V. N. Ipatieff et al., “Synthesis of Methanol from Carbon Dioxide and Hydrogen over Copper-Alumina Catalysts. Mechanism of Reaction”, Journal of the American Chemical Society, 1945, vol. 67, pp. 2168-2171).
Subsequently, other catalysts based on copper-zinc mixed oxides supported on or mixed with aluminum oxides were developed for methanol synthesis. These catalysts are usually prepared by co-precipitation of the corresponding metal oxides in aqueous medium, which enables to control the active surface of the catalyst and its copper content. The methanol production was typically carried out under moderate temperature (220-275° C.), and low-pressure (50-100 bar) conditions. Unfortunately, under the above-mentioned reaction conditions, a per-pass conversion of the process even using a high ratio H2:rich syngas (H2/CO=5) is limited to low total carbon oxides conversion. This low carbon oxides per-pass conversion has been attributed to the thermodynamic limitation of the highly exothermic reaction. Consequently, the recycling of unreacted carbon oxides is necessary to enhance their conversion, leading to a higher production cost associated with a more sophisticated process design.
The high exothermicity of the reaction is also well known to deactivate the catalyst of the reaction in the long term. In this area, Pontzen and co-workers reported a representative example of a process using a commercially available copper-zinc-aluminium oxide based catalyst for the synthesis of methanol by reacting hydrogen with carbon dioxide, at a temperature of 250° C. and a pressure of 70 bar, and a molar ratio of hydrogen to carbon dioxide of 3.1 to 1. This process has a high selectivity to methanol formation but the carbon oxides per-pass conversion proved to be low, since a loop reactor with product separation and internal recycle was required to achieve high overall conversions (F. Pontzen et al., “CO2-based methanol and DME—Efficient technologies for industrial scale production” Catalysis Today, 2011, vol. 171, pp. 242-250).
Another copper-based catalyst has been disclosed with the purpose of improving per-pass methanol yields. A binary catalyst based on the combination of a homogeneous potassium formate catalyst and a solid copper-magnesia catalyst in alcohol solvents was used for methanol synthesis starting from syngas at low temperature (Tian-Sheng Zhao, et al. “A novel low-temperature methanol synthesis method from CO/H2/CO2 based on the synergistic effect between solid catalyst and homogeneous catalyst”, Catalysis Today, 2010, vol. 149, pp. 98-104). Unfortunately, due to the use of a solvent and of a homogeneous catalyst, the methanol production at industrial scale is not easy to realize.
Alternatively, another methanol synthesis under modified reaction conditions, such as supercritical conditions, was performed with the purpose of improving per-pass methanol yields. A copper-zinc based catalyst (Cu/ZnO) was employed for methanol formation under supercritical conditions. The reaction conditions therein disclosed imply a temperature of 270° C., a pressure of 62 bar, and the addition of solvents, for instance n-hexane or 2-butanol. (Prasert Reubroycharoen, et al. “Methanol Synthesis in Inert or Catalytic Supercritical Fluid”, Fischer-Tropsch Synthesis, Catalysts and Catalysis, Studies in Surface Science and Catalysis, 2007, vol. 163, pp. 367-378). Unfortunately, the continuous process for preparing methanol requires a large quantity of the solvent, and thus, the use of the above-mentioned conditions at industrial scale production of methanol is difficult to be implemented.
Additionally, the document US2010088951 describes a process for the synthesis of methanol from synthesis gas, namely a mixture of carbon monoxide and hydrogen with a ratio of hydrogen to carbon monoxide comprised from 0.5 to 1, under a pressure comprised from 35 to 200 bar, at a temperature of 275-300° C., at a space velocity comprised from 2000 to 4000 h−1, and in the presence of a catalyst comprising copper, zinc and potassium.
Similarly, the document U.S. Pat. No. 4,477,594 describes a process for the synthesis of a mixture of methanol and aliphatic hydrocarbons from carbon monoxide and hydrogen in the presence of a catalyst comprising copper, aluminium, zinc, potassium and other metal oxides, under a pressure of 35-200 bar, at a temperature between 200 and 450° C. These processes produce methanol along with other alcohols and by-products with a low carbon oxide conversion, and a low selectivity to methanol formation, as well as a low productivity of methanol.
Therefore, from what is known it is derived that there is still the need of providing a more productive process for the preparation of methanol by reduction of carbon oxides with a high per-pass conversion, high selectivity, and without the deactivation of the catalyst used.